Hydrophobic coatings for glass surfaces are known using traditional silane chemistry. When a glass surface is provided with a hydrophobic coating and the glass is used for a windshield or side window of an automobile, the driver's visual field is secured. Typically, an alkoxylated silane is allowed to react with the glass surface, thereby attaching the coating material to the substrate. Unfortunately, the inherent disadvantage to this approach is that the resulting silicone bond (Si—O—Si) is vulnerable to hydrolysis. Methods to minimize the hydrolysis include increasing the packing density of the material on the surface. This causes steric effects to aid in the prevention of water reaching the reaction site, thereby preventing hydrolysis. These hydrophobic coatings provide improved visibility when used on automobile window glass. However, these coatings have a tendency to wear off within a few months.
The problems of hydrophobic coatings also extend to the airline industry. The current coating technologies are inadequate because of their limited durability. Technologies for improving the visibility in aircraft under rainy conditions include “jet blast” and windshield wipers. Windshield wiper systems further include hydrophobic coatings for greater effectiveness. The “jet blast” system involves blanketing the surface of the windshield with a blanket of high velocity air. However, there is still a need for a more durable coating that will last for more than a year and significantly improve the pilot's visual field during inclement weather.
The above water-repellent glass is generally produced by a wet-coating method in which a water-repellent agent containing an organic silicon compound, typified by a polydimethylsiloxane compound or a fluorine-containing silicon compound, is wet-coated on a glass surface, or by a dry-coating method in which the above water-repellent agent is dry-coated by means of plasma or vapor deposition. However, in the above methods of coating the water-repellent agent directly on a glass surface, it is difficult to maintain water repellency for a long time, since the adhesion strength between the water-repellent agent and the glass is low.
The non-wettability of a substrate, more commonly referred to as its hydrophobic/oleophobic property, consists in the fact that the contact angles between a liquid and this substrate are high, for example at least about 60° for water. The liquid therefore tends to flow readily over the substrate, in the form of drops, simply under gravity if the substrate slopes, or under the effect of aerodynamic forces in the case of a moving vehicle. Examples of agents which are known to impart this hydrophobic/oleophobic property are fluorinated alkylsilanes as described in U.S. Pat. Nos. 5,571,622; 5,324,566; and 5,571,622 which are hereby incorporated by reference in their entirety. According to these patents, this layer is obtained by a solution containing fluorinated organosilanes in a non-aqueous organic solvent is applied to the surface of a substrate. Preferred non-aqueous organic solvents include n-hexadecane, toluene, xylene, etc. These solvents are particularly suitable for a fluorinated chlorosilane. It is also possible to use a methyl or ethyl alcohol as solvent when the fluorinated silane is a fluorinated alkoxysilane.
The hydrophobic coating includes hydrolyzable fluorinated alkylsilanes. They are preferably of the monomolecular type obtained from at least one fluorinated alkylsilane whose carbon chain, which may be branched, comprises at least six carbon atoms, with the carbon of the extremity (extremities) being entirely substituted by fluorine. The layer can also be obtained from fluorinated alkylsilanes or from a mixture of fluorinated alkylsilanes and, possibly, a mixture of fluorinated alkylsilanes and silanes of the SiX4 type in which X is a hydrolyzable group. The current hydrophobic coatings also have environmental and safety issues. The coating may involve the application of perfluoro alkyl silanes in a halogenated hydrocarbon solvent. Current coatings also employ cationic quaternary ammonium compounds and silico-titanium copolymers.
For the foregoing reasons, there is a need for a more durable hydrophobic coating, preferably with a contact angle of at least about 90°, more preferably 100° capable of maintaining acceptable contact angles for at least about 1.5 years. This minimum is extended to about 3 years if the coating is not easily replaceable.